Jan Swammerdam, who demonstrated that an isolated frog muscle could be made to contract when the sciatic nerve was irritated with a metal object, conducted the first muscle experiments between 1661 and 1665. Later, between 1737-1798 Luigi Galvani determined that frog muscle responded to electrical currents. The kymograph, which was invented in the late 1840’s lead to the revolution of experimental physiology because it enabled muscle contractions to be analyzed and recorded. The muscle cell or fiber is the basic unit of a muscle. The frog gastrocnemius muscle contains many muscle fibers. Each of these fibers has its own threshold and responds all-or-none (twitch) when stimulated. All of the fibers in a muscle do not have the same threshold and when a stimulus is applied to the muscle does not necessarily excite all the fibers in it.
The threshold is the intensity of a stimulus, which brings about a response. As the intensity of the stimulus increases above the threshold, more and more fibers are stimulated and the response becomes vast. The strength of the muscle contraction therefore can be increased in two ways: by stimulating existing active motor units and by increasing the number of active motor units. The force that a muscle can generate is dependent upon the total number of muscle fibers. So muscles with small cross-sectional areas are not able to generate large forces as those with large cross-sectional areas.
Muscle fibers cannot continually lift. After a short time, the muscle will lose its ability to shorten and will ultimately fail. Due to the accumulation of waste products and the depletion of stored energy materials, a muscle is said to have lost its contractility and become fatigued. Eventually, stimulus intensity is reached beyond which the response is constant. When this occurs the stimulus is called the maximal stimulus mark, which is the point where all fibers in the muscle are stimulated and responding all-or-none....
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